Waveguides are often constructed by the use of numerous waveguide sections having flanges at their ends to facilitate their connection in series. When high power microwaves are to be carried, cooling ducts can be positioned against the sides of each waveguide sections to carry a cooling fluid such as water. A duct section can be welded to either side of each waveguide section. After the waveguide sections have been connected in series the ducts can be interconnected by hoses extending around the flanges and connecting the ends of adjacent ducts. However, it has been found that power breakdown or arcing, and consequent eroding of the waveguide still occurs near the flanges of the waveguide sections. Temperature measurements have been made at various points along such waveguide sections, which show that a high temperature such as 150.degree. C. may occur at the flanges of waveguide sections carrying microwave power at a level such as 400 kilowatts at the X-band (about 8 gigahertz). A system which minimized hot spots along a waveguide composed of numerous sections connected in series, which could be constructed at low cost, and which did not significantly interfere with rapid connection and disconnection of the waveguide sections, would be on considerable use in high power microwave applications.